Multi-RAT in which heterogeneous communication networks are merged has been researched. For example, a multi-RAT user equipment (UE) supports both a cellular network and a wireless local area network (WLAN). Such a multi-RAT UE can selectively access any one of a plurality of RATs but cannot simultaneously access the plurality of RATs. That is, even when a UE has multi-RAT capabilities, the UE cannot simultaneously transmit and receive data via different RATs.
In a multi-RAT environment, when link with a specific RAT is unpredictably disconnected while a UE accesses the specific RAT and transmits and receives data, the UE terminates access to the specific RA and accesses another RAT. However, there is a problem in that data that has been transmitted and received to and from the specific RAT cannot be seamlessly transmitted and received to and from another newly accessed RAT.